This invention relates to the field of superconductivity. It is a method of construction by which the training effect is eliminated from coils formed of a composite material including both superconducting and normal materials.
The use of superconducting materials for current-carrying coils to generate large magnetic fields offers many theoretical advantages. In particular, the possibility of achieving much higher current densities than are possible in normal materials and the possibility of reducing lost electrical energy both tend to make the superconducting coil an attractive option where large magnetic fields are desired. It is, however, normally necessary to protect superconducting materials against destruction by thermal runaway if the superconductor goes normal by embedding the superconducting material in the form of filaments in a matrix of a material that exhibits higher electrical conductivity in the normal state than do superconducting materials. Copper is the most commonly used material for such a conducting matrix. Aluminum and alloys of copper and nickel have also been used. The matrix provides a protective shunt in the event that the superconducting material becomes normal for any reason. However, a phenomenon known as the training effect interferes with the ready use of composite materials. The training effect is observed in composite superconducting coils as a reversion to the normal state at a value of current far below the design maximum value when current is first applied to the coil. As the current in the coil is cycled between zero or a small value and a large value, the upper limit of coil current is observed to increase continually. It is as though the coil were being trained to carry an increasingly large value of current; thus, the name "training effect". The result is a theoretically unsatisfying, expensive, and potentially dangerous situation in which a superconducting coil must be repeatedly quenched to the normal condition to train it to carry higher values of current. This is time-consuming and risks damage to the coil during the training process.
Compounding this problem is the fact that most superconducting coils are operated in liquid helium at temperatures near 4.degree.K. In order to maintain the superconducting state of all the superconducting material throughout the matrix, it is necessary to assure that no temperature gradient in the matrix causes a local region in which the temperature exceeds the critical temperature of the superconductor. There are two basic ways to keep down temperature gradients in the matrix material. One is to minimize distances between portions of the matrix material and liquid helium by means such as passing cooling channels through the matrix material. This has the disadvantage of reducing the net current density if the total cross-sectional area of material including the area of the cooling ducts is considered as a necessary portion of the cross-sectional area of the coil. This is obviously a situation that is preferably avoided where one is attempting to construct a coil for placement in a limited volume. When this is a consideration, it has been common to use metallic fins of high conductivity as heat drains sandwiched between layers of the composite material to provide better conduction to the liquid helium bath of heat generated in the matrix. The other way to minimize temperature gradients is to use potting compounds that are selected for good heat conductivity.
The training effect is believed to result from a non-uniform residual stress distribution over the cross section of the composite material. The cross-sectional stress distribution is distorted by bending of the superconducting composite material during manufacturing, spooling for transport and forming of the wire into a coil. Test observations are consistent with this hypothesis in that the material exhibits inelastic behavior when subjected to tensile stresses.
It is an object of the present invention to provide a method of construction of composite superconducting coils that minimizes the training effect.
It is a further object of the present invention to provide a method of constructing superconducting coils that causes the generation of a minimum amount of heat in the coils during normal operation.
It is a further object of the present invention to provide a method of construction of superconducting coils that allows the coils to carry the theoretical maximum design current.
Other objects will become apparent in the course of a detailed description of the invention.